1. Field of the Invention
The invention relates to a silver electrode-forming paste composition for a solar cell, a method of producing such a composition, and a solar cell. More specifically, the invention relates to a silver electrode-forming paste composition for forming a silver (Ag) electrode on the light-receiving surface (front surface) or the back surface side of a crystalline silicon-type solar cell.
This application claims priority to Japanese Patent Application No. 2012-159868 filed on Jul. 18, 2012, the entire contents of which are hereby incorporated by reference.
2. Description of the Related Art
Crystalline silicon solar cells, which are solar cells that use crystalline silicon (monocrystalline or polycrystalline) as the semiconductor substrate, are familiar as typical examples of solar cells for converting the energy of sunlight into electrical power. One such crystalline silicon solar cell is the single-side light-receiving type of solar cell 110 shown in FIG. 13.
This solar cell 110 has an n-Si layer 116 formed by pn junction formation on a light-receiving surface (top surface in FIG. 13) side of a p-type silicon substrate (Si wafer: p-Si layer formed of p-type crystalline silicon) 111, and has, on the n-Si layer 116, an anti-reflective coating 114 that is composed of titanium oxide or silicon nitride and front electrodes (light-receiving surface electrodes) 112 that are composed of silver (Ag). The solar cell 110 also has, on a back surface (bottom surface in FIG. 13) side of the p-type silicon substrate (p-Si layer) 111, back side electrodes for external connection 122 that are composed of silver (Ag) as in the light-receiving surface electrode 112, an aluminum electrode 120 that exhibits a back surface field (BSF) effect, and a p+ layer (BSF layer) 124 that is formed by the diffusion of aluminum to the p-Si layer 111.
One method of forming such a light-receiving surface electrode 112 involves forming an anti-reflective coating 114 by chemical vapor deposition (CVD) or the like over substantially the entire surface of the silicon substrate 111, partially removing with hydrofluoric acid (HF) or the like those areas of the anti-reflective film 114 where the light-receiving surface electrode 112 is to be formed, then printing a paste composition composed primarily of a silver powder (silver paste) in the removed areas and firing the printed paste.
Another known method, referred to as the “fire-through method,” involves forming an anti-reflective film 114 over substantially the entire surface of the silicon substrate 111, then applying a silver paste directly onto the areas of this anti-reflective film 114 where the light-receiving surface electrodes 112 are to be formed and firing the applied paste, thereby melting the anti-reflective film 114 below the silver paste and establishing electrical contact between the silver paste and the silicon substrate 111. The silver paste used in this fire-through method is essentially composed of for example, silver powder, a glass component and an organic medium. The glass component within the paste breaks through the anti-reflective film in the course of firing, thereby achieving ohmic contact between the silver ingredient within the paste and the n-Si layer 116. Such a method, compared with an electrode-forming method that entails partial removal of the anti-reflective film 114, is able to reduce the number of operations and presents no risk of mismatching between the removed areas of the anti-reflective film 114 and the light-receiving surface electrode 112 forming positions. Hence, this fire-through method is suitably used to form the light-receiving surface electrodes 112.